1. Field of the Invention
The present invention relates to projection lenses and image projection apparatuses using the same and, more particularly, to projection lenses suited to, for example, liquid crystal projectors of the type in which an original projection image displayed on a display panel, such as a liquid crystal display element, is projected obliquely onto a screen in an enlarged size. Still more particularly, the present invention relates to such a liquid crystal projector that has a projection lens appropriately designed to correct well the distortion and trapezoidal deformation of the projected image and the declination of the image plane so that a projected image of good quality can be obtained.
2. Description of Related Art
Heretofore, there have been proposed a variety of projection apparatuses arranged to project an original projection image displayed on a display panel, such as a liquid crystal display element, onto a screen.
FIG. 21 is a diagram of geometry of the arrangement of a projection apparatus of the oblique projection type in which an original projection image LCD is obliquely projected onto a screen S by a projection lens LP.
In the projection apparatus shown in FIG. 21, for the purpose of preventing the projected image from being distorted when the original projection image LCD is projected on the screen S, the so-called image frame shifting is utilized in which the center LCDa of the original projection image LCD is shifted downward with respect to an optical axis Ax of the projection lens LP.
In the conventional liquid crystal projector, when installed in the apparatus, despite a short projection distance as usual, the projected image has to appear more upper. To this purpose, the projection lens is made wider in the image angle. The center of the liquid crystal panel is thus allowed to take an offset (shifted) position relative to the optical axis of the projection lens, so that the center of the projected image appears upper than the optical axis of the projection lens.
However, when the image angle of the projection lens is widened, distortion, field curvature, lateral chromatic aberration and other aberrations become larger, causing an increase of the difficulty of correcting these various aberrations.
Particularly with the distortion left large, because the projected image is offset from the optical axis, the projected image is distorted asymmetrically in the vertical direction, giving rise to a problem of detracting the quality of the projected image.
To solve this problem, among others, a method of introducing an axially-symmetric aspheric surface into the projection lens, too, has been put into practice. However, since the original projection image LCD is offset as shown in FIG. 22, it is difficult correct the distortion throughout an image circle IC including the entirety of the original projection image LCD.
It is an object of the present invention to provide, by using an aspheric surface having no axial symmetry in a part of an optical system, a projection lens which can correct trapezoidal deformation and distortion as an original projection image is projected obliquely, and an image projection apparatus using such a projection lens.
To attain the above object, in accordance with an aspect of the invention, there is provided a projection lens arranged to project an image formed on a display panel onto a screen, the projection lens comprising a plurality of lens units movable along an axis for varying magnification, wherein at least one of lenses constituting the plurality of lens units has at least one lens surface of shape having no symmetry with respect to the axis.
Further, in the above projection lens, the display panel and the axis are almost orthogonal with each other, and a center of the display panel is offset from the axis.
Further, in the above projection lens, a normal of the display panel makes an angle xcex8 with the axis, wherein the following condition is satisfied:
xe2x88x925xc2x0 less than xcex8 less than 5xc2x0
Further, in the above projection lens, at least one of lenses constituting the plurality of lens units has at least one aspheric surface of shape having symmetry with respect to the axis.
Further, the above projection lens comprises, in order from the screen side, a negative lens unit having a negative refractive power and a positive lens unit having a positive refractive power.
Further, in the above projection lens, the negative lens unit has a lens surface of shape having no symmetry with respect to the axis.
Further, in the above projection lens, the negative lens unit comprises, in order from the screen side, a lens subunit of meniscus form convex toward the screen side, a lens subunit of bi-concave form, and a lens subunit of bi-convex form.
Further, in the above projection lens, the lens subunit of meniscus form has a lens surface, facing the screen side, of shape having no symmetry with respect to the axis.
Further, in the above projection lens, the positive lens unit has an aspheric surface having symmetry with respect to the axis.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.